In general, all superconducting radio-frequency resonating (SRF) cavities, or simply superconducting cavities, require removal of material from their interior surfaces due to the metallurgical damage introduced by forming and welding, and due to contamination that penetrates those interior surfaces. Important metrics of SRF cavity technology are manufacturing yield, the Quality Factor (Q), and the accelerating electric field (EAcc) which increase with increasing amount of removed material, up to a point where the damaged material is removed and the uncovered pristine metal provides the best superconducting properties. These quantities drive cost and performance factors related to cryogenics, beam energy, and machine length. Quality Factor (Q) is proportional to cavity shape (G) divided by surface resistance (Rs), i.e.,QαG/Rswhereby G depends on the cavity shape and Rs depends on cavity surface geometry and chemical purity. The invented method aims to improve performance via optimization of Rs.
Cavity fabrication and processing emphasizes very smooth surfaces because both Q and EAcc can be improved as the surface roughness is decreased. When operated in the usual mode that aligns the electric field with the cavity's longitudinal axis to accelerate a particle beam, magnetic fields align with the cavity surface and are highest near the cell equator. The high magnetic fields seen near the cell equators can cause flux penetration and a local breakdown in the superconducting state that leads to global quenching of the cavity.
SRF cavities are an enabling technology for efficient particle accelerators. They are central to physics machines that produce high-energy and high intensity beams, and they enable other applications such as next-generation light sources, sub-critical nuclear reactors and spent fuel remediation, medical isotope production, emissions reduction, and screening for defense and security.
Niobium is generally considered the metal of choice for SRF cavities, partly because of its temperature stability, corrosion resistance, and because it is paramagnetic. It has the largest magnetic penetration depth of any element. Also it is an unregulated metal. However, the invented method can be applied to other materials too, and particularly those harder than niobium, including but not limited to vanadium, technetium, titanium, copper, aluminum, steel, and alloys thereof.
After SRF cavities are made there is an 80 to 120 micron damage layer covering the inner surface of the cavities which must be removed. The damage layer is most prevalent at the equatorial weld bead produced during manufacture of the SRF cavity. This material has typically been removed in the past by buffered chemical polishing (BCP). The chemical polishing is done in two steps, the first being a “bulk” removal of 100-150 μm of metal. The “bulk” removal step appears to be less important for the final properties. Good results have been obtained when “light” electropolishing (EP) is the second step. EP is used if higher accelerating gradients are required. EP processes produce a typical average roughness (RA) of approximately 0.1 μm for a 1 mm×1 mm area scan using a profilometer.
EP has several drawbacks. The electrolyte that is typically used, 9 parts by volume 98 percent concentrated sulfuric acid and 1 part 49 percent concentrated hydrolluoric acid, is toxic and requires extensive facilities, training, and operation protocols. Spent acid must be disposed of as a hazardous toxin. Sulfur byproducts can form and deposit on the surface, potentially limiting cavity performance. In addition, the complexity of the EP process can make it difficult to control fluorine ion diffusion into the surface of the cavity. Generally, EP requires follow-on high pressure rinses and hydrogen degassing equipment.
Beyond these drawbacks, a central problem with EP is its tendency to retain topographic profiles for features larger than a few tens of micrometers. This means that detects such as weld pits, which can be several hundred micrometers across and 10-100 μm deep, are not removed by bulk EP, even though the topography becomes relatively more smooth. For cavities that are limited by quench at such defects, repeated EP processing often does not repair this situation.
Another problem with EP is that it cannot remove pits near the equator welds of SRF cavities. These pits may be the result of flaws of material preparation prior to welding, flaws in the welding process itself, flaws in the EP process or a combination of these possibilities.
Centrifugal barrel polishing (CBP) is sometimes used to remove the bulk of unwanted film on the insides of SRF cavities. CBP provides a means for imparting pressure on polishing media against the inner surfaces of a cavity. CBP comprises a main shaft rotating in one direction and barrels in rotatable communication with the hub rigidly attached to the main shaft. This allows CBP to rotate SRF cavities residing in those barrels at a direction opposite of the main shaft at typical rotation rates of 100-150 rpms. CBP can create a more uniform surface, albeit initially a rougher one, by ameliorating or minimizing surface defects caused by welding or as a natural characteristic of the work piece.
After undergoing CBP processes, it was still necessary to undergo chemical polishing as a follow up treatment, as previous standard CBP alone generates surface finishes of no better than about 100 nanometers. This is because the mechanical forces that CBP imparts on cavity surfaces often leads to smearing of layers of the surface and pull-out or avulsion of regions of the cavity surface.
In addition to the foregoing, heat treatment (up to 800° C.) is a typical part of cavity surface preparation processes. This heat treatment can cause an additional 20 micron thick layer of contamination in addition to the original unwanted 80-120 micron layer made during cavity manufacturing.
A need exists in the art for a method for producing smooth surfaces for use in SRF. The method should eliminate the use of high concentrations and quantities of acid, and the heat treatment step of state found in the art fabrication techniques. The method also should utilize simple CBP techniques and require only average technician skills.
These and other objects, aspects, and advantages of the present disclosure will become better understood with reference to the accompanying description and claims.
Briefly, the invention provides a method for preparing SRF cavities without using acid, the method comprising selecting a polishing media, loading the polishing media into a SRF cavity, and rotating the cavity such that the polishing media machines the inner surface of the cavity. The machining process by the polishing media is a slow abrasion type action as the polishing media tumbles within the cavity. This Extended Mechanical Polishing (XMP) method produces a substantially defect free surface, with a profile smoothness of less than 15 nm root mean square roughness over approximately a 1 mm2 scan area. Generally, the invented XMP method removes damage from the inner surface of the cavity associated with welding of the cavities during initial fabrication, and has shown being capable of repairing cavities damaged with pits.
A myriad of cavity shapes are accommodated with the invented method. Generally, the invented process can be performed on elliptical cavities of different frequencies. There are many different elliptical shapes, with three exemplary ones being TESLA, reentrant, and low-loss shapes. For example, suitable cavities include those depicted in FIG. 1. Referring to FIG. 1, a cavity 11 when operated in the usual mode that aligns the electric field with the cavity's longitudinal axis 12 to accelerate a particle beam, magnetic fields align with the surface of a cavity 11 and are highest near the cell equator 13. An iris 14 exists between each radially extending portion of the cavity. For polishing operations, the cavity 11 is fitted with a cap 15 to seal the cavity 11.
A feature of the invention is that the SRF cavity surfaces may be prepared without the use of acid treatments. Where acid treatments are desired, for example in the final stage of polishing, approximately 85 percent less acid is required compared to typical cavity surface preparation methods. An advantage of this invention is the elimination of toxic acid waste streams and buffered chemical polishing.
Still another object of the present invention is providing a method for creating or repairing Tesla type cavities. A feature of the method is that the cavities are polished in an inert atmosphere. Another feature is that only mechanical means are utilized to polish the cavities. An advantage of the method is its heightened safety and lack of need for special handling facilities.
Yet another object of the present invention is to produce SRF cavities with high Q and EAcc at a 90 percent or better yield rate. A feature of the invention is that the XMP process is conducted at temperatures between about 20° C. and 50° C. An advantage of the invention is that no chemistry step is necessary to produce the SRF cavities.
Also provided is a method for smoothing inner surfaces of superconducting cavities, the method comprising performing more than one stage where each stage comprises: selecting a polishing media, loading the polishing media into a resonance cavity of a SRF cavity, rotating the SRF cavity such that the polishing media machines an inner surface of the SRF cavity, and evacuating the SRF.